1. Field of the Invention
The present invention is related to a radiation image detector that records radiation images, by generating electric charges when irradiated by radiation and accumulating the generated electric charges.
2. Description of the Related Art
Various types of radiation image detectors that generate electric charges when irradiated by radiation which has passed through subjects, temporarily accumulate the generated electric charges in charge accumulating sections, convert the accumulated electric charges into electrical signals, then output the electrical signals have been proposed, for use in the field of medicine and the like. Various types of radiation image detectors have been proposed. Classified based on the process which is employed to read out the accumulated electric charges, there are radiation image detectors of the optical readout type and radiation image detectors of the Thin Film Transistor type (hereinafter, referred to as “TFT type”). Radiation image detectors of the optical readout type read out the accumulated electric charges by irradiating readout light onto the detectors. Radiation image detectors of the TFT type read out the accumulated electric charges by turning electrical switches such as TFT's ON and OFF, pixel by pixel.
The aforementioned radiation image detectors are equipped with: a photoconductive layer that generates charges when radiation is irradiated thereon; a common electrode to which a predetermined voltage is applied, provided on the side of the photoconductive layer onto which the radiation is irradiated; and a plurality of divided electrodes provided on the side of the photoconductive layer opposite that of the common electrode. An electric field is formed within the photoconductive layer by applying high voltage between the common electrode and the divided electrodes. The electric charges, which are generated within the photoconductive layer due to irradiation of radiation which has passed through a subject, are caused to move to the divided electrodes by the electric field. Radiation is detected by reading out the electric charges which are collected at the divided electrodes as radiation detection signals.
Japanese Unexamined Patent Publication No. 2006-156555 discloses a radiation image detector of the TFT readout type. An insulative film, or an insulative material that includes carbon particles or metallic particles is provided to cover all pixel electrodes of this radiation image detector, in order to improve flatness and film properties.
Japanese Unexamined Patent Publication No. 2005-183670 discloses a radiation image detector of the optical readout type. Insulative films are provided at the edges of divided electrodes of this radiation image detector, in order to prevent dark current from being generated at the edges of the divided electrodes.
In the configuration disclosed by Japanese Unexamined Patent Publication No. 2006-156555, the pixel electrodes are covered by the insulative film. However, the insulative film covers the flat electrode portions of the pixel electrodes, in addition to the edges thereof. Therefore, the charge transport properties are poor, resulting in reduced sensitivity and poor residual image characteristics.
In the configuration disclosed by Japanese Unexamined Patent Publication No. 2005-183670, the edges of the divided electrodes are covered by the protective films. However, as illustrated in FIG. 7, simply providing the protective films results in cracking of a photoconductive layer 2 and deterioration of electrical properties (increase in dark current) at the angular portions that exist at the interface between the protective films 59 and the photoconductive layer 2. An advantageous effect that electric fields which are generated between a common electrode 1 and divided electrodes 5a and 5b are prevented from being concentrated at the edges of the divided electrodes 5a and 5b can be obtained, by providing the protective films 59 at the edges thereof. However, because the protective films 59 are of a uniform thickness, the electric fields which are formed at the divided electrodes 5a and 5b become nonuniform, for example, being greater toward positions Pb toward the edges of the divided electrodes 5a and 5b than at positions Pa at the centers thereof. Accordingly, there is a possibility that image quality will suffer.